In a typical reciprocating compressor, a rotary motor is coupled to a crankshaft that restricts the piston's minimum and maximum displacement. Linear compressors, on the other hand, lack a crankshaft and the piston is driven directly by a linear motor. As a result, linear compressors generally operate more efficiently because the frictional losses resulting from use of a crankshaft are eliminated. Linear compressors are currently used in compression refrigeration systems and a variety of other applications.
When used as part of a compression refrigeration system, linear compressors typically use a linear oscillating driver, which includes an armature mounted between two springs, that drives a piston attached to the armature. The piston reciprocates axially within a barrel and, during a compression stroke, refrigerant is compressed by the piston within the barrel and is discharged through a discharge valve once a preselected compression pressure is reached. The piston then reverses direction during the suction stroke and refrigerant is drawn through a suction valve into the barrel. The compression and suction strokes are repeated at a preselected frequency.
A linear compressor's efficiency is related to the stroke length of the piston. The larger the stroke length, the more refrigerant that can be drawn into and compressed within the barrel during each cycle. A maximum stroke length is thus desirable in order to maximize the compressor efficiency. The absence of a crankshaft to control the maximum displacement of the piston, however, makes it difficult to reliably maintain the maximum stroke length in linear compressors. If the piston travels too far, or “overstrokes”, it can strike the valves or other parts of the barrel assembly during the compression stroke, causing objectionable noise and damage to the valves, pistons or other parts of the compressor over time.
Although it has been suggested that various types of position sensors could be used to detect piston position within the compressor, the use of such sensors has been said to create installation problems by requiring the routing of wires through the walls of the pressurized compressor. An alternative method to detecting and controlling piston position is disclosed in U.S. Pat. No. 5,342,176. The method disclosed in that patent estimates piston position at closest approach to the cylinder head using measurements of motor voltage and current obtained outside the compressor. Those voltage and current measurements are then input to a digital or analog computation device which calculates piston position based on known linear motor properties and known dynamics of piston motion. Because the calculated piston position is subject to errors based on variations in refrigerant system conditions, a need exists for a method to more accurately detect and control overstroking of a linear compressor.